• Rankin Group


When using a portable air conditioner to cool a space, size definitely matters – but bigger doesn’t necessarily mean better. Inaccurately sizing a portable air conditioner is a common occurrence among consumers and such a mistake is detrimental to the effectiveness and efficiency of a unit.

The Goldilocks Principle applies to portable ACs. The consumer needs to choose a size that is “just right” for the intended job. A unit that is too small for a space will be unable to meet the cooling demands and the room will not cool to the desired temperature. An oversized model will cool a room so fast that the thermostat will cycle off before the air is properly dehumidified. This is a significant waste of electricity. It also creates an uncomfortable stuffy and moist environment, and promotes the formation of condensation as well as mold and mildew growth.

The size of a portable air conditioner is described in British Thermal Units (BTUs). The higher a model’s BTU, the bigger its cooling capacity. Most units have a capacity between 5,000 and 12,000 BTUs, but sizes of up to 30,000 BTUs are also available.

In order to appropriately size a portable AC, the square footage of the area that needs to be cooled should be measured. To measure a square- or rectangular-shaped room, the length of the area should be multiplied by its width. For a triangular room, the length should be multiplied by the width and the sum divided in half.

In addition, in a heavily shaded area, capacity should be reduced by 10 percent and in a space absorbing excessive sunlight, capacity should be increased by 10 percent. For rooms regularly occupied by more than two people, 600 BTUs should be added for each person. If a unit is installed in a kitchen, the capacity should be increased by 4,000 BTUs.

Other factors that could cause you to adjust the size of a unit include ceiling height; climate zone; insulation values of the structure; the number, size, and location of windows and skylights; and heat-generating items like lighting, computers, and printers.

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